As the display technology advances, more and more active matrix organic light emitting diode (AMOLED) display panels enter the market. Compared to a conventional thin film transistor liquid crystal display (TFT LCD) panel, an AMOLED display panel has a faster response speed, a higher contrast, and a wider viewing angle. Therefore, AMOLED display panels gain more and more attention of panel manufactures.
FIG. 1 is a circuit diagram of a pixel circuit in an existing AMOLED display panel. It can be seen from FIG. 1 that, the circuit diagram includes a thin film transistor TD, a thin film transistor TS, a storage capacitor C and an OLED. The thin film transistor TS has a gate connected to a scan signal line Vscan, a drain connected to a data signal input terminal Vdata, and a source connected to a gate of the thin film transistor TD. The thin film transistor TD has a drain connected to a cathode of the OLED, and a source connected to a second power supply ELVss, and the second power supply ELVss is at a low level. Two terminals of the storage capacitor C are connected across the gate and the source of the thin film transistor TD. An anode of the OLED is connected to a third power supply ELVdd, and the third power supply ELVdd is at a high level. The thin film transistor TD and the thin film transistor TS may each be an N type thin film transistor.
FIG. 2 is a timing diagram for driving the pixel circuit in FIG. 1. It can be seen from FIGS. 1 and 2 that, in time period t1, the scan signal line Vscan is at a high level, and thus the thin film transistor TS is turned on. At this time, a high level at the data signal input terminal Vdata is written into the storage capacitor C and the gate of the thin film transistor TD, thus the thin film transistor TD is turned on, at this point, the cathode of the OLED is connected to the second power supply ELVss, and the OLED starts to operate and emit light. In time period t2, the scan signal line Vscan is at a low level, and thus the thin film transistor TS is turned off. At this time, due to the discharge retention effect of the storage capacitor C, the gate of the thin film transistor TD remains at high-level state, thus the thin film transistor TD is still on, the OLED continues operating, and the light-emitting state of the OLED may not change until a subsequent time point when a high-level signal of the scan signal line Vscan arrives. It can be seen from the above that, the thin film transistor TS controls the writing of a voltage of the data signal input terminal Vdata, and thus is generally referred to as switch transistor, and the thin film transistor TD controls operating state of the OLED, and thus is generally referred to as driving transistor. In addition, the storage capacitor C mainly plays a role of maintaining voltage.
However, at least the following problems exist in the prior art. The threshold voltage Vth of the driving transistor TD may drift as display time of the panel increases, and luminance of the OLED is closely related to the threshold voltage Vth of the driving transistor TD, so a change in the threshold voltage Vth of the driving transistor TD will have a great impact on the luminance of the OLED, and specifically, the change in the threshold voltage Vth of the driving transistor TD affects luminance uniformity of the OLED. In addition, in a light emission holding phase of the AMOLED display panel, electric leakage of the switch transistor TS will also result in a change in the driving voltage of the gate of the driving transistor TD, and thus lead to uneven light emission of the AMOLED display panel.